Re: [6.2 regression][bisected]discard storm on idle since v6.1-rc8-59-g63a7cb130718 discard=async

Sergei Trofimovich
2023-03-15T16:35:11Z

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On Wed, 15 Mar 2023 12:44:34 +0100
"Linux regression tracking (Thorsten Leemhuis)" <regressions@leemhuis.info> wrote:

> Hi, Thorsten here, the Linux kernel's regression tracker. Top-posting
> for once, to make this easily accessible to everyone.
>
> I added this to the tracking, but it seems nothing happened for nearly
> two weeks.
>
> Sergei, did you find a workaround that works for you? Or is this
> something that other people might run into as well and thus better
> should be fixed?

I used the workaround of cranking up IOPS of discard from 10 to 1000:

# echo 1000 > /sys//fs/btrfs/<UUID>/discard/iops_limit

But I am not sure if it's a safe or a reasonable fix. I would prefer someone from
btrfs to comment if it's an expected kernel's behaviour.

> Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
> --
> Everything you wanna know about Linux kernel regression tracking:
> https://linux-regtracking.leemhuis.info/about/#tldr
> If I did something stupid, please tell me, as explained on that page.
>
> #regzbot poke
>
> On 02.03.23 11:54, Sergei Trofimovich wrote:
> > On Thu, 2 Mar 2023 17:12:27 +0800
> > Anand Jain <anand.jain@oracle.com> wrote:
> >
> >> On 3/2/23 03:30, Sergei Trofimovich wrote:
> >>> Hi btrfs maintainers!
> >>>
> >>> Tl;DR:
> >>>
> >>> After 63a7cb13071842 "btrfs: auto enable discard=async when possible" I
> >>> see constant DISCARD storm towards my NVME device be it idle or not.
> >>>
> >>> No storm: v6.1 and older
> >>> Has storm: v6.2 and newer
> >>>
> >>> More words:
> >>>
> >>> After upgrade from 6.1 to 6.2 I noticed that Disk led on my desktop
> >>> started flashing incessantly regardless of present or absent workload.
> >>>
> >>> I think I confirmed the storm with `perf`: led flashes align with output
> >>> of:
> >>>
> >>> # perf ftrace -a -T 'nvme_setup*' | cat
> >>>
> >>> kworker/6:1H-298 [006] 2569.645201: nvme_setup_cmd <-nvme_queue_rq
> >>> kworker/6:1H-298 [006] 2569.645205: nvme_setup_discard <-nvme_setup_cmd
> >>> kworker/6:1H-298 [006] 2569.749198: nvme_setup_cmd <-nvme_queue_rq
> >>> kworker/6:1H-298 [006] 2569.749202: nvme_setup_discard <-nvme_setup_cmd
> >>> kworker/6:1H-298 [006] 2569.853204: nvme_setup_cmd <-nvme_queue_rq
> >>> kworker/6:1H-298 [006] 2569.853209: nvme_setup_discard <-nvme_setup_cmd
> >>> kworker/6:1H-298 [006] 2569.958198: nvme_setup_cmd <-nvme_queue_rq
> >>> kworker/6:1H-298 [006] 2569.958202: nvme_setup_discard <-nvme_setup_cmd
> >>>
> >>> `iotop` shows no read/write IO at all (expected).
> >>>
> >>> I was able to bisect it down to this commit:
> >>>
> >>> $ git bisect good
> >>> 63a7cb13071842966c1ce931edacbc23573aada5 is the first bad commit
> >>> commit 63a7cb13071842966c1ce931edacbc23573aada5
> >>> Author: David Sterba <dsterba@suse.com>
> >>> Date: Tue Jul 26 20:54:10 2022 +0200
> >>>
> >>> btrfs: auto enable discard=async when possible
> >>>
> >>> There's a request to automatically enable async discard for capable
> >>> devices. We can do that, the async mode is designed to wait for larger
> >>> freed extents and is not intrusive, with limits to iops, kbps or latency.
> >>>
> >>> The status and tunables will be exported in /sys/fs/btrfs/FSID/discard .
> >>>
> >>> The automatic selection is done if there's at least one discard capable
> >>> device in the filesystem (not capable devices are skipped). Mounting
> >>> with any other discard option will honor that option, notably mounting
> >>> with nodiscard will keep it disabled.
> >>>
> >>> Link: https://lore.kernel.org/linux-btrfs/CAEg-Je_b1YtdsCR0zS5XZ_SbvJgN70ezwvRwLiCZgDGLbeMB=w@mail.gmail.com/
> >>> Reviewed-by: Boris Burkov <boris@bur.io>
> >>> Signed-off-by: David Sterba <dsterba@suse.com>
> >>>
> >>> fs/btrfs/ctree.h | 1 +
> >>> fs/btrfs/disk-io.c | 14 ++++++++++++++
> >>> fs/btrfs/super.c | 2 ++
> >>> fs/btrfs/volumes.c | 3 +++
> >>> fs/btrfs/volumes.h | 2 ++
> >>> 5 files changed, 22 insertions(+)
> >>>
> >>> Is this storm a known issue? I did not dig too much into the patch. But
> >>> glancing at it this bit looks slightly off:
> >>>
> >>> + if (bdev_max_discard_sectors(bdev))
> >>> + fs_devices->discardable = true;
> >>>
> >>> Is it expected that there is no `= false` assignment?
> >>>
> >>> This is the list of `btrfs` filesystems I have:
> >>>
> >>> $ cat /proc/mounts | fgrep btrfs
> >>> /dev/nvme0n1p3 / btrfs rw,noatime,compress=zstd:3,ssd,space_cache,subvolid=848,subvol=/nixos 0 0
> >>> /dev/sda3 /mnt/archive btrfs rw,noatime,compress=zstd:3,space_cache,subvolid=5,subvol=/ 0 0
> >>> # skipped bind mounts
> >>>
> >>
> >>
> >>
> >>> The device is:
> >>>
> >>> $ lspci | fgrep -i Solid
> >>> 01:00.0 Non-Volatile memory controller: ADATA Technology Co., Ltd. XPG SX8200 Pro PCIe Gen3x4 M.2 2280 Solid State Drive (rev 03)
> >>
> >>
> >> It is a SSD device with NVME interface, that needs regular discard.
> >> Why not try tune io intensity using
> >>
> >> /sys/fs/btrfs/<uuid>/discard
> >>
> >> options?
> >>
> >> Maybe not all discardable sectors are not issued at once. It is a good
> >> idea to try with a fresh mkfs (which runs discard at mkfs) to see if
> >> discard is being issued even if there are no fs activities.
> >
> > Ah, thank you Anand! I poked a bit more in `perf ftrace` and I think I
> > see a "slow" pass through the discard backlog:
> >
> > /sys/fs/btrfs/<UUID>/discard$ cat iops_limit
> > 10
> >
> > Twice a minute I get a short burst of file creates/deletes that produces
> > a bit of free space in many block groups. That enqueues hundreds of work
> > items.
> >
> > $ sudo perf ftrace -a -T 'btrfs_discard_workfn' -T 'btrfs_issue_discard' -T 'btrfs_discard_queue_work'
> > btrfs-transacti-407 [011] 42800.424027: btrfs_discard_queue_work <-__btrfs_add_free_space
> > btrfs-transacti-407 [011] 42800.424070: btrfs_discard_queue_work <-__btrfs_add_free_space
> > ...
> > btrfs-transacti-407 [011] 42800.425053: btrfs_discard_queue_work <-__btrfs_add_free_space
> > btrfs-transacti-407 [011] 42800.425055: btrfs_discard_queue_work <-__btrfs_add_free_space
> >
> > 193 entries of btrfs_discard_queue_work.
> > It took 1ms to enqueue all of the work into the workqueue.
> >
> > kworker/u64:1-2379115 [000] 42800.487010: btrfs_discard_workfn <-process_one_work
> > kworker/u64:1-2379115 [000] 42800.487028: btrfs_issue_discard <-btrfs_discard_extent
> > kworker/u64:1-2379115 [005] 42800.594010: btrfs_discard_workfn <-process_one_work
> > kworker/u64:1-2379115 [005] 42800.594031: btrfs_issue_discard <-btrfs_discard_extent
> > ...
> > kworker/u64:15-2396822 [007] 42830.441487: btrfs_discard_workfn <-process_one_work
> > kworker/u64:15-2396822 [007] 42830.441502: btrfs_issue_discard <-btrfs_discard_extent
> > kworker/u64:15-2396822 [000] 42830.546497: btrfs_discard_workfn <-process_one_work
> > kworker/u64:15-2396822 [000] 42830.546524: btrfs_issue_discard <-btrfs_discard_extent
> >
> > 286 pairs of btrfs_discard_workfn / btrfs_issue_discard.
> > Each pair takes 10ms to process, which seems to match iops_limit=10.
> > That means I can get about 300 discards per second max.
> >
> > btrfs-transacti-407 [002] 42830.634216: btrfs_discard_queue_work <-__btrfs_add_free_space
> > btrfs-transacti-407 [002] 42830.634228: btrfs_discard_queue_work <-__btrfs_add_free_space
> > ...
> >
> > Next transaction started 30 seconds later, which is a default commit
> > interval.
> >
> > My file system is of 512GB size. My guess I get about one discard entry
> > per block group on each
> >
> > Does my system keeps up with scheduled discard backlog? Can I peek at
> > workqueue size?
> >
> > Is iops_limit=10 a reasonable default for discard=async? It feels like
> > for larger file systems it will not be enough even for this idle state.
> >

--

Sergei

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